The Dental Lamina

Knowledge about the origin, development and fate of the dental lamina is critical to our understanding of how supernumerary teeth arise. In humans, the primitive oral cavity or stomatodeum becomes apparent around day 25 of development and is demarcated laterally by the first pair of branchial (pharyngeal) arches [Krause and Jordan, 1965; Nanci, 2003]. The latter consist of an external layer of ectoderm, an intermediate layer of lateral plate mesoderm and an internal layer of endoderm. Microscopically, the stomatodeum is lined by a two- to three-cell layer of epithelial cells that overlies a layer of embryonic connective tissue derived from cranial neural crest ectomesenchyme. Around day 37 of embryonic development, the presumptive maxilla and mandible become visible as each is covered by a continuous layer of thickened epithelium, called the primary epithelial band. Shortly after, this band of tissue divides into two distinct structures, the vestibular lamina and the dental lamina. While both structures are composed of highly proliferative cells that rapidly grow into surrounding ectomesenchyme, cells of the vestibular lamina enlarge and undergo programmed cell death to form a cleft (the space between the cheek and the arch). In contrast, the dental lamina remains proliferative, extending outgrowths into the ectomesenchyme but only at future sites of odontogenesis. The epithelial outgrowth or tooth bud then progresses from the cap to the bell stages of morphogenesis but remains attached to the dental lamina through a stalk-like extension called the lateral lamina. During the bell stage and prior to the differentiation of odontoblasts and ameloblasts, the dental lamina and the lateral lamina that connects the tooth organ to the overlying oral epithelium fragment into small clusters of cells. These cells normally undergo programmed cell death but sometimes persist to form structures called epithelial pearls. The formation of eruption cysts from these epithelial remnants is known to interfere with the normal eruption pathway of the underlying tooth organ [Moskow and Bloom, 1983; Pindborg, 1970]. Thus far, studies on the fate of the dental lamina have been limited to histologic and ultrastructural analyses that suggest that the sequence of events leading to its degeneration is initiated by underlying mesenchyme [Khaejornbut et al., 1991].

The primary dental lamina that gives rise to the deciduous dentition is also responsible for the formation of the succedaneous (permanent) incisors, canines and premolars. These develop as lingual extensions of the dental lamina and are clearly visible in a coronal section through a developing human jaw. The three permanent molars that

Fig. 1. Panel describing the dental complications and the sequence of treatment for a 17-year-old male affected by CCD. a, b Panoramic radiograph and tracing. CI = Central incisor; L = lateral incisor; C = canine; P = premolar; 6 = permanent first molar; 7 = permanent second molar; 8 = permanent third molar; D = deciduous first molar; E = deciduous second molar. The presence of multiple supernumerary teeth creates several problems. Note that the only permanent teeth which have erupted are the maxillary left central incisor and all first molars. Multiple retained deciduous teeth are also present. c Preoperative intraoral frontal photo

Fig. 1. Panel describing the dental complications and the sequence of treatment for a 17-year-old male affected by CCD. a, b Panoramic radiograph and tracing. CI = Central incisor; L = lateral incisor; C = canine; P = premolar; 6 = permanent first molar; 7 = permanent second molar; 8 = permanent third molar; D = deciduous first molar; E = deciduous second molar. The presence of multiple supernumerary teeth creates several problems. Note that the only permanent teeth which have erupted are the maxillary left central incisor and all first molars. Multiple retained deciduous teeth are also present. c Preoperative intraoral frontal photo graph showing that the majority of permanent teeth are unerupt-ed. d Surgical exposure and extraction of all supernumeraryteeth and deciduous teeth with the exception of the right deciduous second molar, which was not extracted for orthodontic anchorage. e Intraoral view of mandibular arch, showing lip bumper appliance. Orthodontic traction, through bonded attachments with chains, will be applied to promote eruption of the remaining teeth. A series of orthodontic interventions are planned to restore normal occlusion.

do not have deciduous predecessors develop from an outgrowth of the primary dental lamina backward into the jaw ectomesenchyme. There is relatively little known about the genetic factors that control the fate of the dental lamina. Future studies on the patterns of differential gene expression within this specialized epithelium and its surrounding mesenchyme are needed to increase our understanding of the molecular mechanisms that lead to aberrations like missing teeth or the formation of supernumerary teeth.

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